The present invention relates generally to adjustable speed drives (ASDs) and, more particularly, to a system and method for detecting ground faults in an ASD and protecting the ASD from such ground faults upon detection thereof.
Adjustable speed drives (ASDs) are widely used in motor control field for energy efficiency improvement. Such ASDs are typically connected to a three-phase AC power supply, with the ASD including an AC/DC converter for converting three-phase AC power supplied from the three-phase AC power supply into DC power and also including a DC/AC converter for converting the DC power output from the AC/DC converter into three-phase AC power for supply to a motor.
In providing power to a motor via an ASD, it is necessary to be able to detect current faults that might occur and provide protection to the ASD when such faults are detected. A common cause of such current faults is motor winding insulation failures that occur during operation. Such winding insulation failures may cause a winding shorted to the motor grounded enclosure, resulting in a ground fault. When this happens, the shorted phase current on the ASD output phase will rise sharply and, if there is no fault detection and protection in place, the ASD equipment can be damaged. Thus, it can be seen that the detection of current faults and the implementation of a protection scheme for the ASD upon such fault detection, is an important consideration in motor drive applications.
A common solution for detecting current faults is to measure all three-phase currents on the motor side through the use of current sensors. To this end, over-current protection circuitry typically includes a means for monitoring all three-phase motor currents (i.e., three current sensors) and means for shutting off the inverter IGBTs (insulated gate bipolar trasistors) when a current irregularity is identified. When one of the line currents exceeds a predetermined threshold value, the circuitry recognizes the possibility of a short and shuts off the inverter to all three motor phases, effectively stopping the motor until the cause of the irregularity is identified. This solution is very effective in ground fault detection and protection; however, the costs of all three current sensors, as well as the supporting circuitry can be expensive.
As the detection of current faults via measuring all three-phase currents through the use of three current sensors can be expensive, current fault detection has also previously been achieved with the use of only two current sensors. In such systems, the remaining one of the three phase currents is obtained through calculation by assuming that the sum of the three phase currents is zero. Unfortunately, where only two line currents are measured and the third current is derived, the derived current may not reflect a fault to ground in the third line. This is because a connection to ground in the third line may not significantly affect the currents in the first and second lines. If a short occurs in a third line and is not detected because the third line current is derived via the first and second sensed currents, the over-current circuitry cannot operate properly to shut off current to the three phases and motor damage may occur.
It would therefore be desirable to provide a current sensing and protection apparatus and method wherein all currents in a three phase motor system can be derived using less than three current sensors and complete over-current protection can also be provided via the sensed currents and specific voltages, such as the DC link voltage and inverter IGBT saturation voltages.